1. Field of Invention
The present invention relates to a surface acoustic wave device having interdigital electrodes to perform conversion between an electrical signal and a surface acoustic wave. More particularly, the invention relates to a compact surface acoustic wave device adapted to a low frequency.
2. Description of Related Art
Surface acoustic wave devices (hereinafter “SAW devices”) have recently been used as resonators, band-pass filters and the like, for example, in electronic components and communication components of portable telephones, television sets and the like, for example.
FIG. 15 is a plan view showing an example of a related art SAW device.
A SAW device 1 is a single-port type SAW resonator which has a piezoelectric substrate 2, and an interdigital transducer (IDT) 3 and reflectors 4.
The piezoelectric substrate 2 is formed like a rectangular plate from quartz, for example. The IDT 3 and the reflectors 4 are formed in an interdigital configuration using photolithography or the like after forming a conductive metal in the form of a thin film on a top surface of the piezoelectric substrate 2 using vacuum deposition, sputtering, or the like.
Specifically, the IDT 3 is a plurality of electrode digits 3a that are arranged side by side at a predetermined pitch and are formed such that longitudinal ends thereof are alternately shorted. That is, comb-tooth portions of two comb-shaped electrodes are formed such that they are interdigitated with each other at predetermined distances. The IDT 3 performs conversion between an electrical signal and a surface acoustic wave (SAW) through an external terminal 5 that is electrically connected thereto.
The reflector 4 is a plurality of conductor strips 4a that are arranged side by side at a predetermined pitch, and are formed such that both longitudinal ends thereof are shorted. For example, two reflectors 4 having the same configuration are formed such that the conductor strips 4a are formed in parallel with the electrode digits 3a of the IDT 3 and sandwich the IDT 3 at predetermined distances from the each reflectors in a propagating direction of a surface acoustic wave, i.e., a direction orthogonal to the longitudinal direction of the electrode digits 3a of the IDT 3. The reflectors 4 reflect a surface acoustic wave propagated from the IDT 3 and contain the energy of the surface acoustic wave therein.
In such a configuration, when an electrical signal is input to the IDT 3 through the external terminal 5, it is converted into a surface acoustic wave because of a piezoelectric effect. The surface acoustic wave is propagated in a direction orthogonal to the longitudinal direction of the electrode digits 3a of the IDT 3 and directed toward the reflectors 4 from both sides of the IDT 3. At this time, a surface acoustic wave that has a propagating speed determined by the material of the piezoelectric substrate 2, the thickness of the electrodes, the width of the electrodes, and a wavelength equal to an electrode period d0 of the electrode digits 3a of the IDT 3, is most strongly excited. The surface acoustic wave is subjected to multiple reflection by the reflectors 4 to be returned to the IDT 3, and is converted into an electrical signal having a frequency (operating frequency) near a resonance frequency which is then output from the IDT 3 through the external terminal 5.
The above-described related art technique has the following problem.
The recent trend is to make various information apparatuses carrying SAW devices to be very compact. This results in a need to reduce in the size various SAW devices including SAW devices adapted to high frequencies and SAW devices adapted to low frequencies in accordance with the purpose of the apparatus to which they are loaded.
The types of surface acoustic waves utilized for SAW devices include waves referred to as Rayleigh waves and waves referred to as SH waves (share horizontal waves).
Referring to the propagating speeds of those surface acoustic waves, for example, a Rayleigh wave has a speed of 3150 m/sec when propagating in an ST-cut quartz X, and an SH wave has a speed of 5000 m/sec when propagating in a 36 degree rotated Y-cut quartz Y.
Since a SAW device has a frequency f=v/λ (“v” represents the acoustic velocity of the piezoelectric substrate, and “λ” represents the wavelength of the vibrational wave), it has a longer wavelength and greater IDT intervals, the lower the frequency f. Therefore, there is a stronger need for the use of a Rayleigh wave that has a low speed in reducing the size of a SAW device adapted to a low frequency.
The Rayleigh waves and SH waves are different in characteristics as described below.
FIG. 16 illustrates displacement components of a Rayleigh wave in a piezoelectric substrate 2, and the Rayleigh wave has a displacement component U3 in a depth direction of the piezoelectric substrate 2 and a displacement component U1 on a top surface of the piezoelectric substrate 2 in a direction orthogonal to the depth direction when the wave travels in a propagating direction A. The wave as a whole resembles a wave on the surface of the water.
On the contrary, FIG. 17 illustrates displacement components of an SH wave in the piezoelectric substrate 2. While the SH wave is traveling in the propagating direction A, it is substantially occupied by displacement components U2 in a direction orthogonal thereto, which appears as shown in FIG. 18 when viewed from above.
For example, while piezoelectric substrates in which an SH wave and a Rayleigh wave are propagated may use the same quartz, they have different cutting angle orientations and propagating directions which will be described below, as shown in FIG. 19. In the case of the quartz substrate shown in FIG. 19 propagating an SH wave in which the cutting angle orientation (φ, θ, ψ) of the piezoelectric substrate is (0, 126, 90), relative displacement components U1, U2, U3 of the SH wave associated with the coordinate shown in FIG. 20 are as shown in FIG. 21 in which the Z-direction of FIG. 20 is shown in a normalized form with a wavelength along the abscissa axis and the relative displacement is plotted along the ordinate axis.
FIG. 22 similarly illustrates each of relative displacement components U1, U2, U3 in the case of a piezoelectric material to propagate a Rayleigh wave in which the cutting angle orientation (φ, θ, ψ) of the piezoelectric substrate in FIG. 19 is (0, 123, 0).
Thus, in the case of a Rayleigh wave, the displacement U3 in the depth direction Z of the piezoelectric material gradually decreases compared to that of an SH wave.
Further, while in accordance with the related art, an SH wave can be reflected at an end face of the piezoelectric substrate 2 that is perpendicular to the traveling direction of the surface acoustic wave because it is a transverse wave, a Rayleigh wave is not returned because it is transformed into a bulk wave at the end face of the piezoelectric substrate 2 perpendicular to the traveling direction of the surface acoustic wave
While a piezoelectric material that propagates a Rayleigh wave must be used to fabricate a compact SAW device adapted to a low frequency taking the above-described points into consideration, a reflector is indispensable in this case in order to contain energy because the surface acoustic wave cannot be reflected at an end face of the substrate.
In addition, since the surface acoustic wave must be reflected by the reflector with sufficient efficiency, the reflector must be formed with a great number of conductor strips, and a piezoelectric substrate having a sufficient surface area must be used accordingly. This limits the compactness of the device.